PM123 comes with a skin utility (SKINUTIL.EXE), which can converts WinAmp skins to PM123 skins. Here's an example how to convert a WinAmp skin (SKIN.ZIP):
[c:\pm123] md skin
[c:\pm123] cd skin
[c:\pm123\skin] unzip -j c:\files\skin.zip
Archive: c:/files/skin.zip
inflating: Viscolor.txt
inflating: Position.bmp
inflating: Main.BMP
inflating: Text.BMP
inflating: option.bmp
inflating: Cbuttons.bmp
inflating: Titlebar.BMP
inflating: Monoster.bmp
inflating: Posbar.bmp
inflating: Shufrep.bmp
inflating: Volume.BMP
inflating: Numbers.BMP
inflating: Playpaus.bmp
[c:\pm123\skin] cd ..
[c:\pm123] skinutil convert skin
PM123 Skin Utility (c) 1998 Taneli Leppä <rosmo@sektori.com>
Processing skin/cbuttons.bmp.......
Processing skin/titlebar.bmp..........
Processing skin/text.bmp: abcdefghijklmnopqrstuvwxyz_-()&.:0123456789öä ,+%[]
Processing skin/posbar.bmp..
Processing skin/volume.bmp...
Processing skin/shufrep.bmp:....
Processing skin/monoster.bmp:..
Processing skin/numbers.bmp:............
Processing skin/main.bmp:.
Creating skin.dat...
Creating skin.pak
Bundling...
124 images, total 212444 bytes (207 kB).
Creating skin.skn
Cleaning up...
There you go. You can now load skin.skn from PM123. The skin utility
also created files skin.pak and skin.dat that are required
if you use the skin. You can also bundle your old skins with lots of small
files into two files. Run skinutil.exe without any parameters
to see help.